This project has focused on a small piece of engrailed DNA,2 Kb in length, which shows a number of remarkable properties. First, when inserted in a transposable element (P transposon), it enables the transposon to be inserted specifically next to genes that are expressed in a striped pattern (targeted insertion). Second, the same DNA can extinguish expression of a linked marker gene, white. Transgenic flies containing white transgenes normally have colored eyes, however when the engrailed DNA is present, it silences white gene expression. Third, the targeting aspect can be used to express a therapeutic gene by the regulatory signals flanking it and thereby rescue a lethal phenotype. Gene therapy is in its infancy in terms of approaches, in that most have coupled a gene without the normal regulatory flanking sequences to insert into the human genome. The studies done within this project suggest that a randomly inserted gene might be extinguished or give improper expression. As the field develops, understanding the mechanisms of controlling expression will become more and more paramount. Furthermore, the present studies suggest areas of concern that are not being addressed in current studies such as that different pieces of DNA can alter insertion properties of vectors which insert into the genome, such as retroviral vectors. Understanding why certain fragments of DNA lead to selective insertion could improve future gene therapy vectors making them both more safe and effective.